Topline
Sweat carries sodium, not just water. Here is when replacing it matters, what the sodium concentration data shows, and why most people need electrolytes far less often than marketing suggests.
For sessions under about an hour in comfortable conditions, water is sufficient and an electrolyte drink adds nothing but flavour and cost. Past roughly ninety minutes of continuous work, in heat, or across repeated sessions in a day, sodium replacement starts to matter, and past three or four hours it matters a great deal. That is the whole distinction, and almost everything sold on the strength of the word electrolyte is aimed at people on the wrong side of it.
The reason has nothing to do with the mystique the term carries. Electrolytes are simply minerals that carry an electrical charge in solution: sodium, potassium, chloride, calcium, magnesium. They set the osmotic gradients that determine where water sits in the body, and they are the substrate for nerve conduction and muscle contraction. Losing them in quantity has consequences. Losing a small amount and replacing it at your next meal has none.
What sweat actually contains
Sweat is dilute plasma, but not proportionally dilute. Sodium and chloride dominate; potassium, magnesium and calcium are present in far smaller quantities.
Sodium concentration in sweat varies widely between individuals, typically reported across a range from around 20 to 80 millimoles per litre, with some outliers higher. This spread is genetic to a large degree and only partly modifiable. Heat acclimatisation reduces it: over a week or two of repeated heat exposure, the sweat glands become more efficient at reabsorbing sodium, so an acclimatised athlete loses less salt per litre than the same athlete did on day one. Fitness has a smaller effect in the same direction.
Multiply that concentration by sweat volume and the totals become substantial. A cyclist losing 1.5 litres an hour at a middling sweat sodium concentration is shedding something in the region of 1.5 to 2 grams of sodium per hour, which is a meaningful fraction of a day's intake. The same athlete on a forty-minute easy ride in cool weather loses a small fraction of that, replaced without thought by an ordinary evening meal.
Potassium losses are much smaller in absolute terms and rarely the limiting factor during exercise, though potassium matters considerably for blood pressure and overall diet quality, which is a separate question addressed in our micronutrient guide.
Why plain water can become the problem
The mechanism that makes sodium replacement necessary in long events is counterintuitive. It is not that you run out of salt. It is that replacing sweat losses with plain water dilutes the sodium remaining in your blood.
Plasma sodium concentration, not total sodium content, is what the body regulates and what determines whether water moves into or out of cells. Drink a large volume of sodium-free fluid while sweating heavily and the concentration falls. Mild dilution produces nothing noticeable. Substantial dilution produces exercise-associated hyponatraemia, which presents as headache, nausea, confusion and bloating and, at the severe end, as cerebral oedema.
Almond and colleagues quantified how common the mild form is in the New England Journal of Medicine (2005), sampling Boston Marathon finishers and finding that a substantial minority crossed the line with blood sodium below the normal range. The strongest predictors were gaining weight over the course of the race, which means drinking more than was lost, and a slower finishing time, which means more hours of opportunity to do so.
This is why fluid guidance is framed around limiting losses rather than maximising intake. The ACSM Position Stand on exercise and fluid replacement, in Medicine and Science in Sports and Exercise (2007), sets the goal as preventing body-mass loss beyond about 2% while also avoiding weight gain during exercise. Both boundaries matter. Diagnosis and treatment of hyponatraemia are clinical matters, and the symptom overlap with dehydration makes self-assessment unreliable during an event; anyone unwell during or after prolonged exercise needs medical assessment rather than a guess.
When replacement genuinely matters
The variables that push you toward needing sodium are duration, heat, sweat rate and repetition. A rough hierarchy:
Sessions under an hour in temperate conditions need water and nothing else, regardless of intensity. The sodium lost is small and dietary intake covers it comfortably.
Sessions of one to two hours, or shorter sessions in real heat, sit in a grey zone. Adding sodium is unlikely to help a fit person in normal conditions but becomes reasonable if you sweat heavily, if the environment is hot and humid, or if you notice salt crusting on skin and clothing after training.
Sessions beyond two hours, especially with high sweat rates, are where sodium replacement earns its place. So does any situation involving repeated sessions in a day, consecutive days of heavy training in heat, or a first exposure to a hot climate before acclimatisation has developed.
Beyond exercise, a few non-athletic situations warrant attention: substantial fluid loss from vomiting or diarrhoea, work involving prolonged heat exposure, and certain medications that alter sodium or potassium handling. The last of these is firmly a clinician's territory. Diuretics, some blood pressure drugs and several psychiatric medications interact with fluid and electrolyte balance in ways that make general advice inappropriate.
Sizing the fluid target first
Sodium is a modifier on top of a fluid plan, not a substitute for one. Our water intake calculator builds a daily figure from body weight, then adds for training and climate. For a 70 kg adult in a temperate climate, the baseline works out at 2,450 ml of total water. Add an hour of training and it rises to 3,050 ml. Take that same hour into very hot conditions and it reaches roughly 3,660 ml, of which about 2,930 ml would come from drinks and the remainder from food.
| Scenario (70 kg adult) | Total water | Of which from drinks |
|---|---|---|
| No training, temperate | 2,450 ml | 1,960 ml |
| 1 hour training, temperate | 3,050 ml | 2,440 ml |
| 1 hour training, very hot | 3,663 ml | 2,930 ml |
| 90 min training, hot | 3,718 ml | 2,974 ml |
These are planning figures with a wide error band, built on an average sweat rate that may not be yours. The measurement that replaces them takes ten minutes: weigh yourself unclothed before and after an hour of training, add back the weight of anything you drank, and each kilogram lost corresponds to roughly a litre of sweat. Repeat it in cool and hot conditions and you will have your own range, which is worth more than any population average.
What to actually use
Once you have decided sodium replacement is warranted, the specification is unremarkable. Commercial sports drinks generally supply somewhere in the region of 400 to 600 mg of sodium per litre, which suits moderate losses over moderate durations. Dedicated endurance products go substantially higher, into the region of 1,000 mg per litre and beyond, aimed at heavy sweaters in long events.
Carbohydrate is often bundled in, and for good reason in events past about ninety minutes, where fuelling matters independently of hydration. Glucose also modestly enhances intestinal sodium and water absorption through the sodium-glucose cotransport mechanism, which is the physiological rationale behind oral rehydration solutions. For sessions where you do not need the fuel, sodium without the sugar is a reasonable choice.
Nothing about this requires a branded product. A pinch of table salt in a water bottle delivers sodium at a fraction of the cost, if less pleasantly. Salted food before or after a long session works equally well. So do the ordinary salty items athletes have used for decades. The marketing framing that positions electrolyte products as a daily wellness necessity rather than a situational tool for prolonged sweating is not supported by anything in the physiology.
Magnesium deserves a specific note because it is heavily promoted for cramping. The evidence that exercise-associated muscle cramps are caused by electrolyte depletion is weaker than commonly assumed; the competing explanation, that cramps arise from neuromuscular fatigue and altered reflex control, has substantial support and better explains why cramps cluster in the muscles being worked hardest late in an event. Some athletes with high sodium losses do report benefit from sodium supplementation, and that observation is worth taking seriously in individual cases, but a general recommendation to supplement magnesium for cramp prevention overstates what the trials show.
The daily-diet picture is different
Everything above concerns acute replacement around exercise. The chronic picture inverts almost completely.
The Institute of Medicine's Dietary Reference Intakes for Water, Potassium, Sodium, Chloride and Sulfate (2005) noted that typical intakes in Western diets substantially exceed physiological requirements for sodium, largely from processed food rather than the salt cellar, while potassium intakes fall well short of the recommended level in most of the population. The public health problem is not a shortage of electrolytes in the diet; it is an imbalance between two of them.
The correction is dietary rather than supplemental. Potassium is abundant in vegetables, fruit, pulses, dairy and potatoes, and diets built around those foods reliably deliver more of it while incidentally containing less sodium. This is the logic of the DASH dietary pattern and it is one of the better-evidenced relationships in nutrition.
The apparent contradiction, that an endurance athlete may need deliberate sodium while the general population needs less, dissolves once you separate the timescales. Replacing two grams of sodium lost in a long hot session is a different question from a habitual daily intake that runs well above requirement.
Practical summary
Water covers you for most training. Add sodium when sessions run long, when it is hot, when you sweat heavily or salt visibly, and when you train more than once in a day. Measure your own sweat rate rather than trusting a formula, including ours; the calories burned calculator can help you gauge session load, but heat and humidity drive sweat production more than intensity does.
Do not drink so much during prolonged exercise that you gain weight, and do not treat clear urine as a target. Build the daily background from food rather than sachets, with plenty of vegetables and fruit for potassium. And treat any persistent symptom, cramping that will not resolve, dizziness, confusion during exercise, or a suspicion that a medication is affecting your fluid balance, as a reason to see a clinician rather than to buy a supplement.
For the underlying question of how much fluid you need in the first place, our guide to daily water requirements sets out the reference values and where they come from.